Off-Mesh Links are used to create paths crossing outside the walkable navigation mesh surface. For example, jumping over a ditch or a fence, or opening a door before walking through it, can be all described as Off-mesh links.
We’re going to add an Off-Mesh Link component to describe a jump from the upper platform to the ground.
Now you have functioning Off-Mesh Link set up! If the path via the off-mesh link is shorter than via walking along the Navmesh, the off-mesh link will be used.
You can use any game object in the scene to hold the Off-Mesh link component, for example a fence prefab could contain the off-mesh link component. Similarly you can use any game object with a Transform as the start and end marker.
The NavMesh bake process can detect and create common jump-across and drop-down links automatically. Take a look at the Building Off-Mesh Links Automatically for more details.